Synthetic aperture (SA) imaging can be used to increase resolution beyond the diffraction limit of a physical aperture of an imaging system. In SA imaging systems, a large “virtual” aperture is synthesized by illuminating a target region with electromagnetic signals transmitted from a moving platform, and by collecting phase-coherent return echoes produced by reflection of the electromagnetic signals from the target region. The return echoes are recorded and combined to reconstruct a high-resolution image of the target region. SA imaging was initially developed and has been employed at radio and microwave frequencies, so that the devices in which SA imaging was first implemented were referred to as “synthetic aperture radar” (SAR). Conventional SAR systems typically operate in the centimeter (cm) wavelength range and produce images with azimuth resolutions of the order of a decimeter (dm) to a meter (m). As resolution generally varies inversely to the imaging wavelength, there has been an interest to extend SAR to shorter wavelengths. In this context, an emerging technology referred to as “synthetic aperture ladar” (SAL) has been developed to extend SAR to the visible and near-infrared regions of the electromagnetic spectrum.
SA imaging systems provide two-dimensional (2D) SA images representing projected ground surface reflectance. A 2D SA image can be represented as a complex-valued array of pixels, so that each pixel has an amplitude value and a phase value. The 2D SA image has an along-track dimension measured in azimuth coordinate and an across-track dimension measured in slant-range coordinate. For a non-flat target region, an ambiguity exists in determining the pair of ground-range and elevation values that corresponds to a measured slant-range value. That is, several combinations of ground-range and height values may lead to a same slant-range value. An approach to remove this ambiguity and enable three-dimensional (3D) imaging is known as “interferometric SA imaging”, referred to as IFSAR and IFSAL depending on the operating wavelength. In this technique, two 2D SA images of a target region are acquired from two different viewpoints. The 2D SA images are coco-registered and interfered with each other, and an elevation map of the target region is extracted from their phase difference. A challenge in implementing interferometric SA imaging is that the height reconstruction process involves phase unwrapping, which can suffer from robustness limitations. This is especially true in the case of IFSAL, since the requirements on phase accuracy and platform stability for interferometry tend to become increasingly stringent as the wavelength decreases. Although IFSAL has been demonstrated in laboratory settings, for short target distances and high ground-range resolution, it has yet to be successfully implemented in the field and for low ground-range resolutions. A difficulty encountered with IFSAL is achieving sufficient phase coherence between the two 2D SA images to combine them into a high-quality interferogram. Challenges therefore remain in the field of 3D SA imaging.